The invention relates to a resilient foot insert for an artificial foot.
A jointless prosthetic foot with a resilient foot insert which takes up and transmits the prosthesis loads is disclosed, for example, in DE 40 37 928 A1 and EP 0 648 479 A1. Elastic foot inserts are also to be found in U.S. Pat. Nos. 4,959,073, 5,549,711 and 5,800,570, and in EP 0 884 033 A2 and DE 298 20 904 U1.
The object of the invention is to develop a resilient foot insert for a prosthetic foot which has a progressive ankle moment profile, stores energy and permits an elastic ML (medial-lateral) mobility.
According to the invention, this object is achieved by a resilient foot insert for an artificial foot, consisting of at least two springs which are connected to one another and which together in side view, in the unloaded state, enclose an approximately triangular spring deflection space, the upper, approximately roof-shaped spring having, in the roof top area, an adapter attachment an, starting from the latter, a heel branch (hereinafter xe2x80x9cheel springxe2x80x9d) which extends downward in a concave curve into the heel area, and a forefoot branch (hereinafter xe2x80x9cforefoot springxe2x80x9d) which extends downward in a concave curve in the forefoot area, the free branch ends being connected to a separate, flexurally rigid base spring which delimits the bottom of the spring deflection space and which interacts with a rolling contour in such a way that, at the start of the loading of the forefoot during the rolling action (i.e., walking in a heel-to-toe fashion), the force is first introduced in the are of the ball of the foot, with forefoot spring and base spring being configured in terms of shape and flexural elasticity in such a way that, under the effect of an increasing load in the forefoot area, the forefoot spring and base spring successively bear against one another in this are by respective bending.
Thus, according to the invention, this foot insert is characterized by the forefoot spring and base spring being configured in terms of shape and elasticity in such a way that, with increasing load in the forefoot area, the forefoot spring and base spring come to successively bear against one another in this area. By means of this successive bearing on the base spring, the forefoot spring effects a progressive ankle moment profile. At the start of loading of the forefoot, the force is first introduced in the area of the ball of the foot, by which means the base spring is mounted between heel spring and forefoot spring and experiences three-point bending. There is then a serial connection of the flexural strengths of forefoot spring and base spring (on this point see also the view in FIG. 6). In the further course of the step, the foot effects a rolling action and increasingly loads the forefoot spring directly. The base spring additionally supports the moments in the forefoot spring and is now bent in the opposite direction. There is a parallel connection of the flexural strengths (on this point see also the view in FIG. 7). The course of the loading of the forefoot is thus marked by an increasing stiffening or progression of the spring behavior.
The contact points of the prosthetic foot with the ground or shoe determine the site of introduction of force. This has an influence on the biomechanics of the foot, but also controls the loading of the mechanical structures of the foot. Since the foot experiences an angle movement during the stance phase, the point of introduction of force can be controlled by a suitable rolling geometry. It does not matter whether this geometry is integrated into the configuration of the base spring (see FIG. 4 for example), applied to the base spring (see FIG. 5) or is a feature of the surrounding cosmetic covering (see FIG. 1). In all these cases, the base spring interacts on the underside with a rolling contour which slopes downward from the toe area to the ball area, then slopes upward again, and is shaped as a downward bulge formation in the heel area.
To achieve the effect which is sought, it is also expedient if the front free branch end of the forefoot spring is connected to the front end of the base spring, and if the rear free branch end of the heel spring is connected to the rear end of the base spring.
The characteristics of the foot can be deliberately modified by using base springs of different degrees of rigidity. For this purpose, the connections between the base spring and the two branch ends of the upper spring are made releasable.
ML movement can be generated by reducing the torsional rigidity of the forefoot spring. This can be realized by means of the forefoot spring having an oblong recess extending in its longitudinal direction, or by the forefoot spring being split into two part-springs in its longitudinal direction. In this way it is also possible to set different progression characteristics for the inside face and outside face of the foot. The instantaneous center of the ML rotation lies in the physiologically desirable manner above the ground plane, whereas rotation about the longitudinal axis of the leg is hardly possible.
At least one of the springs is preferably made of a polymer material, which is expediently a fiber composite. With a fiber composite, the introduction of force into the springs can be enhanced if the springs are designed individually, especially if parts of the adapter are a component part of one of the springs. For this reason, it is therefore expedient if forefoot spring and heel spring form separate structural parts which are connected to one another in the area of the adapter attachment. It is of further advantage here if at least the spherical cap of the adapter attachment is an integral component part of the forefoot spring. A construction which is particularly favorable in respect of transmission of force is obtained if the forefoot spring, in the area of its adapter attachment, is supported on the upper branch end of the heel spring.
If the resilient foot insert is to be used in cases where the amputation levels are very low, particularly in cases of disarticulation of the ankle joint (on this point, see also the view in FIG. 9), it is expedient if the adapter attachment is positioned anteriorly of the ankle area of a natural foot and has an attachment surface enclosing an acute angle with the base spring. In this way, the space under the prosthesis shaft can be used as a spring deflection space for the heel branch. In the case of this attachment, the moments on the adapter between loading of the heel and loading of the forefoot are more balanced and lower than in the case of an attachment in the ankle area. The necessary structural strength is thus easier to achieve.